Today has been mostly prepping loo 11 of 10 for the plasterer arriving later this week. First job is to explore what lies behind the mystery window, which involves taking the sashes out. They came out remarkably easily (though it helped having seen professional sash restorers do it, including finding the 'secret door' to the weight channels).
Interestingly the upper and lower weights were different, one set '8' (lbs I assume), the other pair "9 1/2".
The 'wall' behind the window cleaned up quite nicely and so the current plan is to install a mirror there and replace the restored sashes. First off though is to make a fresh panel for one side that wet rotted away some decades ago.